Splitting a House into Flats: Costs and Consents

Converting a house into two or more self-contained flats can increase its value and create separate homes that can be sold or rented.

However, conversion involves building work, planning permission and legal changes. You need to check whether the finished flats justify the investment.

Does Your Property Suit Conversion?

Not every house is suitable for conversion. Each flat needs enough space, natural light and practical access to work as a separate home.

Terraced houses can be difficult because they are often narrow and have one staircase. Creating a separate entrance for an upper flat may also reduce living space. Semi-detached and detached houses can be easier because side access may allow another entrance. 

Large Victorian and Edwardian houses are often suitable because they usually have larger rooms and higher ceilings.

• Independent entrances: Each flat should have its own practical entrance from outside.
• Kitchens and bathrooms: Each unit needs enough space for its own kitchen and bathroom, with suitable plumbing and drainage.
• Utilities: Check whether gas, electricity and water can be supplied and metered separately.
• Space: Each flat should provide enough living space and meet relevant space standards.
• Bins and parking: Check local requirements for bin storage and parking.

The Consents You Will Need

Planning Permission

Splitting one house into two or more flats normally requires full planning permission. Councils may consider parking, privacy, bin storage, external changes and the loss of family housing.

Building Regulations

These cover areas including fire safety, sound insulation, ventilation and energy efficiency. Sound insulation can be a significant cost because existing floors and ceilings may need upgrading to separate the flats properly.

Listed Buildings and Conservation Areas

Extra restrictions may apply to changes such as new doors, windows, entrances or external staircases.

Freeholder and Lender Consent

If the property is leasehold, you may need the freeholder’s permission. Your mortgage lender must also agree.

Party Wall Agreements

Structural work involving shared walls may require notice to neighbours under the Party Wall etc. Act 1996. A party wall surveyor may also be needed.

What Does Conversion Cost?

For a typical two-flat conversion, budget for:

• Architect and planning drawings: Around £2,000 to £6,000.
• Planning application: Around £250 to £600, depending on the application and local requirements.
• Structural engineer: Around £800 to £2,000 for structural advice and calculations.
• Building control: Around £600 to £1,500.
• Party wall surveyor: Around £1,000 to £2,500 for each surveyor involved.
• Fire safety work: Around £6,000 to £15,000 for measures such as fire doors, alarms and compartmentation.
• Sound insulation: Around £4,000 to £10,000 to improve separation between flats.
• Second kitchen: Around £6,000 to £15,000.
• Additional bathroom: Around £4,000 to £9,000.
• New entrance and external work: Around £3,000 to £10,000.
• Separate utilities: Around £3,000 to £8,000 for new supplies and meters.
• Electrical and plumbing work: Around £8,000 to £20,000 to separate or upgrade systems.
• Legal work: Around £2,000 to £5,000 for new leases and titles.
• Contingency: Keep around £5,000 to £12,000 for unexpected costs.

A straightforward two-flat conversion may therefore cost between £50,000 and £120,000. Three-flat projects and conversions in London or the South East can cost considerably more.

Utility separation is often underestimated. New gas, electricity or water connections can involve network charges and long waiting times, so check these requirements early.

Measurement, Lease Plans and the Legal Split

The property must be legally divided as well as physically converted.

Each new flat will usually need its own leasehold title. Leases registered with HM Land Registry need suitable plans that clearly show each property’s boundaries.

A lease plan should use a stated metric scale, show a north point and clearly identify the area included in the lease. An unclear or inaccurate plan can delay registration.

Accurate floor areas are also important for valuations, mortgage applications and marketing. Incorrect measurements could cause a buyer or lender to question the property’s value.

A professional measured building survey can provide accurate floor plans, elevations and sections. These can support the design process and be used to prepare lease plans and confirm floor areas.

You will also need to decide how ownership will work. A common arrangement is to retain the freehold and grant long leases for each flat. Legal documents should explain responsibility for shared areas, maintenance, insurance and the roof.

Does the Financial Case Work?

Compare the combined value of the finished flats with the current value of the house and the full conversion cost.

For example, a house worth £320,000 might become two flats worth £195,000 each, giving a combined value of £390,000. If the conversion costs £85,000, you would make a £15,000 loss before finance, fees and tax.

If each finished flat were worth £220,000 instead, their combined value would be £440,000. After allowing for the original £320,000 property value and £85,000 conversion cost, the potential gain would be £35,000 before other expenses.

This is why you should research recent selling prices for comparable flats nearby rather than relying on estimates.

Other factors can affect the calculation:

• Finance: Standard residential mortgages may not fund major conversion work. Development or bridging finance can cost more, with interest adding up during construction.
• Tax: Capital Gains Tax may apply, particularly if the property is not your main home. Some qualifying conversion work may be eligible for a reduced VAT rate.
• Mortgageability: Very small flats, units without independent access and some flats in mixed-use buildings can be harder to mortgage.
• Timeline: Planning can take several weeks, while construction may take four to eight months or longer.

When Selling Whole Makes More Sense

Conversion is not always the best option. Selling the house without completing the project may make more sense when the expected profit is small, borrowing is too expensive or planning permission is uncertain.

It may also be more practical if the property is inherited, part of a divorce settlement or needs to be sold quickly.

Partly converted houses can be difficult to sell because buyers must take on the cost and risk of finishing the project. This can reduce demand and lead to lower offers.

If certainty and speed are more important than achieving full market value, a cash house buyer may be another option. A cash-buying company may purchase the property in its current condition and offer a fixed completion date without a property chain.

However, cash buyers generally offer less than full market value. Compare the offer with the likely result from an estate-agent sale before deciding.

Conclusion

Splitting a house into flats can work well when the property is suitable, local flat values are strong and planning permission is achievable. However, the project only makes financial sense when the combined value of the finished flats comfortably covers the property’s existing value, construction costs, professional fees, finance and tax.

Before committing, check the planning position, calculate realistic costs and research comparable flat prices in the local area. Make sure each proposed flat has enough space, suitable access and practical services.

You should also budget for unexpected work and allow enough time for planning, construction and legal changes. Careful research before starting will help you decide whether conversion offers a worthwhile return or whether selling the house as it stands is the better choice.